Electrical power is typically delivered to households and other properties using an electrical distribution grid as three-phase power. In a household, some high-power devices, such as ovens, stoves and hot-water heaters utilize all three phases and the smaller devices only one or two of the three phases, typically through the internal power network of the household. Although the internal power network of a household is designed in such a way that all phases are utilized, the load between the phases is in practice seldom even at a given moment of time. That is, there is a load unbalance between the phases.
Decentralized systems for energy production are becoming increasingly common. Such systems typically include renewable energy generator units such as e.g. wind power plants and photovoltaic (solar) power systems, which are typically coupled to the internal electrical network of a household using an inverter, for example. Such systems reduce the need for energy purchased from the distribution grid. However, in particular in the case of a one-phase inverter system, the load balance/unbalance between the three phases in the household's internal network is significantly affected when power is available from the decentralized energy source.
In some countries, it is also possible to feed power further into the electrical distribution grid if excess energy is produced by the renewable energy source. In some countries there is no such possibility due to technical limitations of electricity meters in the switchboards or due to restrictions by the authorities or power distributors. Even if such feed-in is possible, it is usually not economically profitable for the parties involved without the support of the state through feed-in-tariff policies and mechanisms. To avoid or minimize the need for feed-in of energy to the grid, it is preferred to use as much of the renewable energy produced at the household or some other facility. Feeding power to the distribution grid may also result in load unbalances between phases in the distribution grid, which may be harmful for the distribution grid and is therefore undesirable from the power supplier's point of view.
US 2011/298292 discloses a method for feeding three-phase current to a power grid and a solution to the load unbalance problem in the distribution grid in the particular case of a three-phase photovoltaic inverter system as the decentralized energy source. The solution is based on providing a central control and monitoring unit comprising a load unbalance detection function and a load unbalance control function allowing individual photovoltaic inverters to be entirely disconnected from the power grid by means of disconnectors as a result of a control command signal from a monitoring unit. The system is not applicable to a one-phase inverter system such that the power of the energy source would be utilized it the best possible way.
WO 2011/089181 discloses a method of leveling partial powers at a grid connection point between a multiphase power grid and a multiphase inverter unit. Neither this method can be used in a one-phase inverter system.
US 2011/210608 discloses a single stage power conversion system including a photovoltaic source and an inverter for delivery of power to a power grid. In addition, there is a load balancing unit coupled to the inverter and a controller configured to determine a maximum power point for the inverter system, regulate an output voltage of the inverter, compute a power balance difference between a power demand from the power grid and an output power of the inverter obtainable at the maximum power point and control in real-time the load balancing unit based on the power balance difference. The load balancing unit may be an energy storage unit or power dissipation unit. The system is aimed at optimizing power balance difference between a power demand from the power grid and an output power of the single stage inverter. The system does not relate to or solve the problem of load unbalance between individual phases of the power network the photovoltaic source is connected to.
There is a need for improved systems and method for coupling decentralized power production units to common electrical networks.